The present invention relates generally to an apparatus and method of non-destructive evaluation of materials, and more particularly, to an apparatus and method of processing optical information to detect ultrasonic surface displacements through the use of at least one laser to perform a non-destructive evaluation of a material.
In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in the manufacturing processes. Specifically, non-destructive evaluation (xe2x80x9cNDExe2x80x9d) methods are required to assess the structural integrity of composite structures, for example, to detect inclusions, delaminations and porosities. Conventional NDE methods, however, are very slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One method to generate and detect ultrasound using lasers is disclosed in U.S. Pat. No. 5,608,166, issued Mar. 4, 1997, to Monchalin et al. (the xe2x80x9c""166 Patentxe2x80x9d). The ""166 Patent discloses the use of a first modulated, pulsed laser beam for generating ultrasound on a work piece and a second pulsed laser beam for detecting the ultrasound. Phase modulated light from the second laser beam is then demodulated to obtain a signal representative of the ultrasonic motion at the surface of the work piece. A disadvantage associated with this approach is that the first pulsed laser beam must be modulated. Other U.S. Patents issued to Monchalin et al. and relating to the subject matter of ultrasonic material testing include the following:
Although these patents describe operable techniques for optically detecting transient motion from a scattering surface, which techniques are useful for ultrasonic composite materials non-destructive test and evaluation, these techniques have numerous failings.
To begin, none of the Monchalin and other known techniques provide the ability to perform with high signal-to-noise-ratios (SNR) at large distances from typically very dark composite materials using small aperture high-speed optical scanning methods. The ability to operate in such a mode has the distinct advantage of increasing the optical scan area coverage and providing substantially improved depth-of-field thereby eliminating the need for active focusing mechanisms.
Other known techniques do not posses the desirable feature of removing common-mode noise from the laser signals using a fully self-referenced interferometric configuration that uses all of the available light without the use of separate stabilization measurements.
Another limitation associated with the Monchalin and other known apparatuses relates to their inability to operate at very high scan rates and process ultrasonic data in real-time. This limitation makes such apparatuses only marginally useful for testing and evaluating composite materials.
Other limitations associated with existing apparatuses relate to general inflexibility of such apparatuses, which may hold all distances low, result in small depth of field performance and only minimal extraction of information from the back scattered signals. These limitations make industrial application of the ultrasonic testing method generally impractical.
The present invention provides an apparatus and method for generating and detecting ultrasonic surface displacements on a remote target that substantially eliminates or reduces disadvantages and problems associated with previously developed laser ultrasonic systems and methods.
More specifically, the present invention provides a method and system for generating and detecting ultrasonic surface displacements on a remote target. The system includes a first pulsed laser to generate a first pulsed laser beam. The first pulsed laser beam produces ultrasonic surface displacements on a surface of the remote target. A second pulsed laser generates a second pulsed laser beam coaxial with said first pulsed laser beam to detect the ultrasonic surface displacements on the surface of the remote target. Collection optics to collect phase modulated light from the second pulsed laser beam either reflected or scattered by the remote target and optionally optically processed to increase the light intensity. An interferometer to process the phase modulated light and generate at least one output signal. A processor for processing the at least one output signal obtains data representative of the ultrasonic surface displacements on the surface of the remote target.
In another embodiment, a method for ultrasonic laser testing in accordance with the invention comprises using a first pulsed laser beam to generate ultrasonic surface displacements on a surface of a remote target. A second pulsed laser beam coaxial is used with the first pulsed laser beam to detect the ultrasonic surface displacements on the surface of the remote target collecting phase modulated light from the second pulse laser beam either reflected or scattered by the remote target also occurs, processing the phase modulated light to obtain data representative of the ultrasonic surface displacements on the surface of the remote target.
A technical advantage of the present invention is that a method for ultrasonic laser testing is provided. The personal invention provides rapid, non-contact, and non-destructive inspection techniques that can be applied to complex composite structures. The present invention provides a flexible, accurate and cost effective method for inspecting complex composite structures. The present invention is able to rapidly scan and test large-sized composite structures. The present invention is able to inspect at angles off normal (i.e., up to xc2x145 degrees). The present invention does not require expensive fixturing to test composite structures. The present invention does not require the shape of the part to be known prior to testing. The present invention does not require access to both sides of a composite structure to test it for defects.